Alarm system

ABSTRACT

An alarm system in which the ground conductor of the building is used to transmit a current from a detecting unit to a sensing unit which is at the fuse box or service entrance of the electrical system for the building. The detecting unit includes a switch which connects the hot wire of the wiring system to the ground conductor via a high-impedance load so that some current flows through the ground wire. The sensing unit operates an alarm when it detects current flow through the ground wire. The system eliminates the need for separate wiring from each detector unit to the central sensing unit and any room in the building or dwelling can be provided with a detecting unit simply by plugging the detecting unit into an electrical outlet. An indicator on each detecting unit provides a visual indication of the correct ground wiring and the proper connection of the detecting unit to the wiring.

United States Patent Allen, Jr.

[ 1 Feb. 22, 1972 [54] ALARM SYSTEM Therron J. Allen, Jr., 7432 Westwood Park Lane, Falls Church, Va. 22046 [22] Filed: Dec. 19,1969

[21] Appl.No.: 886,669

[72] Inventor:

[52] US. Cl ..340/2l3, 340/310, 340/227.l,

340/216 [51] Int. Cl. ..H04rn 11/04 [58] Field of Search..... ..340/2l3, 252 P, 310, 253 R,

Primary Examiner-Donald J. Yusko Assistant Examiner-Robert J. Mooney AttorneyWalter Kruger [57] ABSTRACT An alarm system in which the ground conductor of the building is used to transmit a current from a detecting unit to a sensing unit which is at the fuse box or service entrance of the electrical system for the building. The detecting unit includes -a switch which connects the hot wire of the wiring system to the ground conductor via a high-impedance load so that some current flows through the ground wire. The sensing unit operates an alarm when it detects current flow through the ground wire. The system eliminates the need for separate wiring from each detector unit to the central sensing unit and any room in the building or dwelling can be provided with a detecting unit simply by plugging the detecting unit into an electrical outlet. An indicator on each detecting unit provides a visual indication of the correct ground wiring and the proper connection of the detecting unit to the wiring.

26 Claims, 6 Drawing Figures PATENTEUFEBZZ I972 SHEET 2 OF 2 HIGH /NTEN$/TY LOW INTEMS/TY 1M FNTOR. fi/ERRONJ Hus/v, JR.

ALARM SYSTEM This invention relates generally to alarm systems and particularly to alarm systems which use a portion of the usual wiring of a building to transmit a signal indicative of a dangerous condition to actuate an audible alarm or other warning device.

More specifically, the invention relates to an alarm system for homes which includes detecting units adapted to be plugged directly into the electrical receptacles of the unusual house wiring, and which when actuated transmit a signal along the code-required ground conductor of the housing wiring, which signal is detected by a sensing unit adjacent the fuse box of the house. This ground conductor can be the ground wire of a nonmetallic cable system, the armored sheath of a B-X type cable system, or the thin-walled conduit of a conduit-type wiring system, these systems representing the only presently approved systems of the National Electrical Code. Since at least one of these systems complies with the local electrical code of any area in the United States, the system of this invention has extensive utility.

Each of the dangerous condition detecting units includes a switch and a load connected between the hot line of the house wiring and the ground wire. The sensing unit is located adjacent the fuse box of the house and is so associated with the ground conductor that it senses current flow through this conductor. Hence, when one of the detecting units is actuated the hot wire is connected to the ground conductor via a load, (to prevent blowing a fuse), and the current flowing through the ground conductor is detected by the sensing device. The load can be a low-voltage load, such as a small 60-volt bulb, which will burn out in several seconds, but which will cause a current to flow in the ground wire for a sufficient time to actuate the sensing device.

The dangerous condition detecting units can be of the type to sense virtually any dangerous condition in the home or building where the system of this invention is installed. For example, the sensing unit can be a burglar-detecting device of the proximity switch type, an unauthorized entry detecting device which is actuated when a window or door is opened, or can be a smokedetecting or thermal switch type detecting unit to detect smoke or fire. Alternatively, it is contemplated that combination of these detecting units can be connected to the system of this invention to warn the occupants of a home or other building about any desired condition which is felt to be dangerous.

In one preferred embodiment, the invention takes the form of a fire alarm with a temperature-sensitive switch in the detecting unit connected between the hot wire and ground wire at a receptacle. The switch is in series with a permanent or short-life load, which draws perhaps l50 watts, to assure the flow of sufficient current in the ground wire to actuate the current-sensing device at the fuse box. When the current-sensing device is actuated, the alarm is connected to a power source which causes the alarm to sound, thereby warning the occupants of the building of the existence of sufficient heat at one of the switch units to actuate the alarm.

A distinct advantage of applicants system is that extensive additional wiring is not required to install the fire alarm system. It is merely necessary to plug a detecting unit into a receptacle of each room of the house where the dangerous condition is to be detected. Depending on the wiring system of the house, it may also be necessary to separately ground one side of a detecting unit at the receptacle.

At the fuse box, a minor modification is necessary. This minor modification includes connecting all the ground conductors of the various receptacle circuits of the building to a common conductor having a current-carrying capacity at least as great as the entrance wiring, which in turn is grounded to the fuse box. The current-sensing device is then positioned adjacent the current conductor, and current flow through this conductor is indicative of actuation of one of the detecting units, which occurs when there is a fire, or other dangerous condition at some location in the building. The audible alarm which warns the occupants of the dangerous condition may be located adjacent the fuse box, or if the house is large, several alarm units can be located in different parts of the house to assure that the occupants will hear the warning signal when the alarm is actuated,

In one preferred embodiment of this invention the current conductor at the fuse box is the primary winding of a transformer. This primary winding includes only several turns of very heavy wire, such as 4 or 6-gauge wire to provide for proper grounding of the ground conductors of the building. The secondary winding of the transformer is connected to the sensing device, and current flow in the secondary, induced by current flow through the primary, actuates the sensing device and alarm system.

In one embodiment of the alarm system of this invention, a battery is provided at the sensing unit and the unit is arranged to sound the alarm if there is a power failure. In this embodiment, the battery is normally charged by the household current and the alarm is so arranged that power from the battery is automatically available to sound the alarm in the event of a power failure. This power failure responsive alarm is a distinct advantage since the house wiring may be damaged by a fire or some other dangerous condition before the alarm is sounded, and hence, an alarm is assured as soon as power is lost. This system also assures that an alarm will sound if an intruder disconnects the powerline to the house where the system is installed.

In accordance with another feature of this invention, a lowlevel alarm is initially sounded for a predetermined period of time to warn the members of the household about the dangerous condition. After the low-level alarm has sounded for this period of time, a high-level alarm, of sufficient intensity or volume to apprise the entire neighborhood of the dangerous condition, is sounded. It will be appreciated that the intensity of the high-level alarm is sufficiently great that even very sound sleepers will be awakened, and members of the household will hear the alarm even when there is a high noise level in the dwelling. By providing a predetermined time interval during which only the low-level alarm is sounded, a member of the household can immediately disable the alarm and then apprise the other members of the household that the alarm has sounded. Then, in the event that the sounding of the alarm was the result of faulty wiring, or some other minor problem, the household is not subjected to the higher intensity second alarm.

The flexibility of the system of this invention will be appreciated by realizing that virtually any type of dangerous condition detector can be simply and inexpensively provided in each room of the house by connecting the detector to the electrical receptacle. Hence, the detecting device may take the form of any type of burglar alarm such as the electric eye type, pressure switch type, or window or door type to protect the household against burglars. In addition, a smoke detector of any of several well-known types, for example, the electric eye type, can be connected to a ceiling receptacle of the household in any desired room to warn about smoke from a smoldering fire. Alternatively, inexpensive plug-in type thermal detectors, constructed in accordance with this invention, can be simply and inexpensively provided in each room of the house, so the household will be warned about a fire regardless of where the tire occurs.

correspondingly, it is an object of this invention to provide an inexpensive alarm system which can be readily installed in a residence with existing wiring, as well as during initial wiring of the residence.

Another object is an inexpensive alarm system which utilizes the code-required ground system of the residence to transmit dangerous condition indicating signals from detection units to a central sensing unit.

Another object is an alarm system including an alarm connected to the powerlines of a building, the alarm having a battery, and circuitry responsive to a power failure to automatically sound the alarm.

A further object is an alarm system which also warns the homeowner of dangerous conditions in the electrical wiring of the residence.

A further object is a fire or other dangerous condition alarm system employing extremely inexpensive detecting units which connect directly to the plug type electrical receptacles of the residence and transmit signals indicative of the dangerous condition along the ground conductor of the residence to a sensing unit remote from the detecting unit.

A further object is an alarm system in which the detecting units each include a low current-carrying load which causes a low current to flow through the ground wire of the residence, which is sufficient to actuate the alarm, but insufficient to either blow the fuses or cause injury to the members of the household.

A further object is a fire or other dangerous condition alarm system in which the low current-carrying load of the detecting units is of the type which burns out in several seconds with household voltage applied, so there is only a short interval of time during which current flows through the ground conductor to actuate the alarm, and correspondingly, there is little if any danger of electrical shock to members of the household.

A further object is a fire alarm system including inexpensive detecting units to warn the occupants of dangerous conditions by operation of a central alarm system, and in which the detecting units include an indicator to indicate the condition of the detector as well as the condition of the wiring of the residence.

A still further object is an alarm system of the type described which initially sounds a low-level alarm, and subsequently sounds a high-level alarm, but not until after the expiration ofa period of time sufficient to allow a member of the household, who immediately hears the low-level alarm, to turn the alarm off, and thus avoid subjecting the household to the high-level alarm.

Numerous other advantages and features of this invention will become apparent with reference to the accompanying drawings, which form a part of this specification and in which:

FIG. 1 is a schematic of a house wiring system including the alarm of this invention;

FIG. 2 is a plan view in section of one of the dangerous condition detecting units of the invention;

FIG. 3 is a schematic of the wiring for the detecting unit of FIG. 2;

FIG. 4 is a schematic wiring diagram of a second embodiment of sensing and alarm units of the invention;

FIG. 5 is a schematic wiring diagram of a second embodiment of detecting unit according to the invention; and

FIG. 6 is a diagrammatic view of a transformer via which grounding conductor can be grounded to the service entrance housing.

Referring now to the drawings in detail and particularly to FIG. 1, there is shown a first embodiment of the alarm system of this invention connected to a conventional household wiring system. The alarm system includes an alarm l, a ground conductor current-sensing device 2, and a plurality of dangerous condition detecting devices 3.

The household wiring system 4 includes a service panel or fuse box 5 to which the usual service entrance wires 6-8 are connected. Wires 6 and 8 are customarily designated hot" wires, whereas wire 7 which connects to neutral strip 9 of the fuse box is designated the neutral wire. Neutral strip 9 is grounded as at 9 in the usual manner, for example, by connecting neutral strip 9 to a water pipe or ground rod.

As shown at FIG. 1. the voltage between wires 6 and 7 or wires 7 and 8 is 120 volts AC, whereas the voltage across wires 6 and 8 is 240 volts AC. Wires 6 and 8 are connected respectively to the hot conductors 10 and 11 of-the fuse box via the main fuses l2 and 13. There is no fuse between neutral strip 9 and neutral wire 7, this unfused arrangement being required by the electrical code. Only the lighting portion of the wiring system of the household is shown at FIG. 1; the portions of the wiring system to supply electric ranges, dryers, and

ovens are not shown to simplify explanation. This lighting portion of the house wiring supplies electricity for lighting and also supplies electricity to the various plug-type receptacles located throughout the household.

As shown at FIG. 1, the wiring system for the household includes a first branch circuit 14, and a second branch circuit 15. Circuit 14 includes a hot" wire 16 connected to the hot" conductor 11 via a fuse 17, and a neutral wire 18 connected directly to neutral strip 9. Wires 16 and 18 are within the sheath of nonmetallic sheath cable 19, known in the trade as romex. Cable 19 also includes a grounding wire 20. While, for purposes of explanation, a house wiring system using nonmetallic sheath cable 19 will be described herein, the alarm system of this invention can readily be used with B-X cable by insulating the metallic cable jackets from the fuse box and using an auxiliary ground conductor, and likewise, the system can be used with conduit-type wiring by similarly insulating the conduits from the fuse box and again using an auxiliary ground conductor to obtain the code required grounding of the conduit at the fuse box.

Branch system 15 includes a hot" wire 21 connected to conductor 11 via a fuse 22 and a neutral wire 23 connected to neutral strip 9. Wires 21 and 23 extend through the sheath of a nonmetallic sheath cable 24, identical to cable 19 and also having a grounding wire 25. Cable 24 extends to the desired portion of the household and is connected to a receptacle 26 housed within the usual outlet box 27. As shown, neutral wire 23 connects to one side of receptacle 26 and hot" wire 21 connects to the other side of receptacle 26, whereas grounding wire 25 connects to the ground terminal 27 of the receptacle. Where outlet box 27 is metal, grounding wire 25 is also connected to the outlet box as at ground tab 26'. The usual electrical conductor arrangement (not shown) is provided within receptacle 26 to connect terminal 27 and hence, grounding wire 25 to the ground socket 28 of the receptacle. Wires 21, 23 and 25 extend from outlet box 27 to another outlet box 29 which, in the usual house wiring system would have another receptacle like receptacle 26, but which is not shown for purposes of simplification.

Nonmetallic cable 19 extends to outlet boxes 30 and 31. At outlet box 31 is another receptacle 32 to which neutral wire 18, hot wire 16, and grounding wire 20 are connected in a manner identical to that described for receptacle 26. Another receptacle (not shown) is at outlet box 30.

In accordance with one embodiment of this invention, an auxiliary ground conductor 35 is connected to fuse box 5. Auxiliary conductor 35 takes the form of a bar or heavy wire, is elongated, and is preferably formed from copper or other metal ofgood electrically conducting characteristics. Conductor 35 is electrically connected to the metal housing of fuse box 5, which in turn is grounded as shown at 35. Extending around conductor 35 is a current-sensing coil 36 of sensing device 2. In this embodiment of the invention, the sensing device 2 advantageously takes the form of a very sensitive latching relay with normally open contacts 37. Contacts 37 are in series with conductor 38 which has one end connected to hot" wire 16 at 39 and its other end connected to one side of alarm 1. The other side of alarm 1 is connected to neutral wire 18 by wire 41. So long as no current flows through conductor bar 35, contacts 37 of sensing device 2 remain open and alarm 1 is silent. However, when there is current flow through conductor bar 35 of sufficient magnitude to close contacts 37 (which occurs when one of detecting devices 3 is actuated), the contacts close and remain closed and alarm 1 emits a warning signal until the contacts 37 are reset.

Contacts 37 are in series with conductors 38 and 39. Conductor 38 connects to one side of alarm 1 and conductor 39 connects to one prong of a plug 40 of the usual household type. The other side of alarm 1 is connected to plug 40 via wire 41. Plug 40 is plugged into the usual type electrical receptacle 42, which as shown at FIG. 1, is located at fuse box 5 and has wires 43 and 44 connected respectively to receptacle 42. It will be appreciated, however, that the plug 40 can be plugged into the electrical receptacle at any convenient location in the household, so long as wires 38, 39, and 41 are sufficiently long to extend to contacts 37 of sensing device 2.

Alternatively, and in accordance with an embodiment of this invention yet to be described, alarm 1 can be battery powered in such a manner that failure of the household power supply is effective to automatically energize the alarm system to warn the household, should a dangerous condition cause a power failure.

When the wiring system of the household is properly connected, and is functioning properly, there is no current flow through any of the ground wires, such as and of cables 19 and 24. Correspondingly, there is no current flow through conductor bar 35, and hence, contacts 37 remain open and no alarm is sounded. However, when there is current flow through conductor bar of sufficient magnitude for sensing coil 36 to close contacts 37, these latching contacts close and remain closed and alarm 1 emits a warning signal until the contacts 37 are manually reset. In accordance with this invention, when a dangerous condition, such as a fire, occurs somewhere in the household, detecting device 3 functions to connect a hot wire to one of the ground wires via a load or to close contacts 37, but insufficient to damage the fuses, flows from the appropriate ground wire through conductor bar 35.

Plugged into the receptacle of each room in the household where it is desired to be able to detect a dangerous condition, such as a fire, is a detecting unit 3. As shown at FIG. 2, detecting unit 3 has a box-shaped housing of molded plastic or other electrically insulating material. Fixed to and projecting through a wall of the housing are plug-type prongs 51 and 52, which are spaces apart the same distance as the plug-receiving openings of the household receptacles. In addition, there is a ground prong 53, also fixed to and projecting from the housing, which aligns with and enters the ground socket, such as ground socket 28 of receptacle 26 when prongs 51 and 52 are plugged into the receptacle. Connected to prong 51 is a conductor 54 formed from springy sheet metal, such as brass or bronze. Conductor 54 is generally U-shaped and has an end in the form of a leg 55 which extends generally parallel to front wall 56 ofthe housing.

A pushbutton 57 of electrically insulating material has an enlarged base 58 and a reduced diameter end 59 which extends through an opening 60 in front wall 56. Leg 55 of conductor 54 engages the inside face of base 58 to normally urge the pushbutton against the inside surface of wall 56, to the position shown at FIG. 2. A conductor bar 61 is fixed to front wall 56 and extends across and parallel to the end of leg 55. Leg 55 forms a movable contact and engages the end 62 of the conductor bar when pushbutton 57 is depressed. Leg 55 and end 62 cooperate to provide a switch 62.

Mounted on the other end 63 of conductor bar 61 is a neon bulb 64 with one of its terminals 65 electrically connected to bar 61. The other terminal of bulb 64 is electrically connected, by a wire 66', to a contact 66 secured to sidewall 67 of the housing 50. Contact 66 is mounted on a threaded stud 68 which extends through sidewall 67 and is secured by a nut 69. Extending from contact 66 is a terminal 70 to which wire 66 from bulb 64 is connected. Also connected to terminal 70 is a wire 71 electrically connected to a terminal bar 72 to which ground prong 53 is connected.

A movable contact arm 73 of springy metal has one end connected to a mounting block 74 fixed to sidewall 67, and has a contact 75 at its free end. A ball 76 oflow melting point material seats in an opening 77 in sidewall 67 and is maintained in this position by the spring bias of contact arm 73. Were it not for ball 76, the spring force of contact arm 73 would move contact 75 into engagement with contact 66. Contact 66, contact arm 73, contact 75, and ball 76 cooperate to provide a thermally responsive switch 78.

Connected between the inner end of prong, 51, and contact arm 73 is a load 79 which draws lO-50 watts of power when connected across a l20-volt AC powerline.

FIG. 3 shows the circuit arrangement of sensing unit 3 schematically. As shown, switch 62' and neon bulb 64 are connected in series across plug prong 51 and ground prong 53. Load 79 is in series with thermal switch 78, and the load and switch are connected across plug prong 51 and ground prong 53, in parallel relation to bulb 64 and switch 62'.

As shown at FIGS. 1 and 3, when sensing unit 3 is plugged into receptacle 26, plug prong 51 is electrically connected to hot wire 21, and ground prong 53 is electrically connected to ground wire 25. Prongs 51-53 support sensing unit 3 at receptacle 26.

Alternatively, if the house where the system of this invention is to be installed is of the older type not having receptacles including a ground socket to receive ground prong 53, a wire 71 (shown in dotted lines at FIG. 2) can be connected to stud 68 of detector unit 50, and this wire is then connected directly to the outlet box with which the receptacle is associated to provide the necessary ground conductor return. Where the receptacle does not have a ground socket, ground prong 53 and wire 71 are, of course, removed.

With sensing unit 3 plugged into receptacle 26, depressing pushbutton 57 closes switch 62' and completes a circuit from prong 51 through conductors 54 and 61 to one terminal of bulb 64. Since the other terminal of bulb 64 is connected to ground prong 53 via conductors 66' and 71, a circuit is complete from prong 51 to prong 53. Since prong 53 connects to ground wire 25, when the detector is plugged into receptacle 26, and since prong 51 connects to hot" wire 21, bulb 64 lights. The bulb will only light if ground wire 25 is properly grounded at fuse box 5. Hence, depressing pushbutton 57 allows the homeowner to test the ground wiring of the house as well as the several connections at receptacle 26. However, because neon bulb 64 draws only a very low current, there is insufficient current through ground wire 25 to conductor bar 35 to cause coil 36 to close contacts 37.

If a fire occurs in the room where detector 3 is plugged into receptacle 26, the heat from the fire causes ball 76 to melt, thereby allowing contact arm 73 to move contact 75 into engagement with contact 66. When contacts 66 and 75 are closed, a circuit is complete from ground prong 53 through conductor 71, contacts 66, 75, contact arm 73, load 79 and prong 51, so that hot wire 21 is electrically connected to ground wire 25 via the load 79. This causes sufficient current to flow through the grounding wire and conductor bar 35 to cause sensing coil 36 to close contacts 37. The alarm sounds and the homeowner is warned about the fire.

Load 79 can be a permanent load in the form of a resistor rated at at least 10 watts, (for a 120-volt system) so a continuous current flows through conductor bar 35 when switch 78 closes. Alternatively, load 79 can be a short-life or momentary load, such as a 60-volt, 10-watt lamp bulb, which will conduct a small current to the conductor bar for only several seconds after switch 78 closes, and will then burn out. The advantage of the momentary load is that the grounding conductors or wires of the building carry a small current for only a short period of time. However, since the relay, including contacts 37 and coil 36, is of the latching type the alarm sounds continuously until contacts 37 are reset.

With reference to FIG. 4, there is shown a more sophisticated embodiment of the sensing and alarm sounding unit of this invention. The sensing and alarm unit 82 has a power supply 82 including a stepdown transformer 83 connected to the plug 84 and wires of a line cord. An ON-OFF switch 84 provides for selective manual control of power to the alarm unit. One side of the secondary winding of transformer 83 is connected to a chassis ground 85 and the other side of the secondary winding is connected to the anode of a rectifier diode 86. Connected to the anode of rectifier diode 86 is a resistor 87. Connected from chassis ground 85 to the anode of rectifier diode 86 is a capacitor 88. A capacitor 89 is connected between the opposite end of resistor 87 and the chassis ground. Capacitor 88, resistor 87, and capacitor 89 cooperate to provide a ripple filter to smooth the half wave rectified current from rectifier diode 86.

The cathode ofa diode 90 is connected to hot line 91 that extends from the junction of resistor 87 and capacitor 89. The positive terminal 92 of battery 93 is connected to the anode of diode 90 and the negative terminal 94 of battery 93 is connected to the chassis ground. A trickle charge resistor 95 is connected in shunt across diode 90.

The components of the sensing unit 82 thus far described form the power supply 82' for this sensing unit. Because of the ripple filter including capacitors 87, 88 and 89, a positive voltage, with some ripple, but which never decreases to zero is available at line 91. A small current flows through resistor 95 to maintain battery 93 charged whenever the alarm unit is connected to the powerline and switch 84 is closed. During normal operation of the power supply circuit, the positive voltage at line 91 is slightly higher than the terminal voltage of battery 93. So long as the positive voltage at line 91, (and correspondingly at the cathode of diode 90) is more positive than the battery terminal voltage, diode 90 is nonconducting and hence, battery 93 cannot discharge. However, in the event ofa power failure, the voltage at line 91 drops to 0.

Then, the anode-cathode voltage across diode 90 becomes positive and is sufficient to cause diode 92 to conduct, with the result that a voltage from battery 93, very slightly lower than the normal voltage at line 91 appears between line 91 and the chassis ground.

A reset switch 96 and an SCR 97 are connected in series with line 91. Gate 98 of the SCR is connected to cathode terminal 99 via a series circuit including, respectively, a capacitor 100, resistor 101, sensing coil 102, and diode 103. A load resistor 104 is connected between chassis ground 85 and a line 105 from cathode terminal 99 of the SCR. Resistor 104 assures that sufficient holding current will pass through the SCR to maintain it conducting after it is triggered by an appropriate signal at gate 98. Connected between line 105 and chassis ground 85 is a low-intensity alarm unit 106. Also connected between line 105 and chassis ground 85 is a time delay relay 107 having a heating element 108 and normally open thermal sensitive contacts 109 in series with a high-intensity alarm 111 which in turn is connected to the chassis ground 85.

Connected from a junction 112 between the secondary winding of transformer 83 and the anode of diode rectifier 86 is the energizing winding 113 of a relay 114. The normally closed contacts 115 of relay 114 are connected in shunt across SCR 97. Contacts 115 are normally closed, but are held open by current through winding 113, which is present whenever alarm unit 82 is plugged into a power supply and switch 84' is closed.

The sensing coil 102 comprises a plurality of turns of wire and is wound on the ground conductor bar 35, previously described with reference to FIG. 1.

OPERATION OF ALARM-SENSING UNIT 82 In use of alarm-sensing unit 82, coil 102 is placed to encircle bar 35, FIG. 1 (alarm 1 and sensing unit are not used), and the coil 102 is connected in the gate circuit of the SCR 97, as shown at FIG. 4. Plug 84 is inserted in a receptacle and switch 84 is closed. Then, reset switch 96 (which is advantageously key operated to prevent tampering) is closed and the alarmsensing unit 82 is in a ready" condition.

In the ready condition, rectifier 86 provides half wave rectified current pulses which are smoothed by the ripple filter including capacitors 88, 89 and resistor 87. Hence, a positive voltage is present at line 91. The several components of the power supply are so selected that the voltage at line 91 is about 14 volts. Since coil 13 is relay 114 is energized, contacts 115 are open and no current is available to energize alarms 106 and 111. In the ready" condition SCR 97 is in a nonconducting state.

When there is a fire or other dangerous condition, and the switch 78 of detecting unit 3 closes, current flows through conductor bar 35. The alternating current in bar 35 induces alternating current in coil 102, which applies a trigger signal to gate 98 of SCR 97 when diode 103 conducts, and the SCR is triggered into conduction. As soon as SCR 97 conducts, current flows through resistor 104 to the common chassis ground, and there is sufficient current flow to maintain the SCR conducting, i.e., the direct current flow through SCR 97 is above the holding current for the SCR. Hence, once the SCR is con ducting the direct current, it remains conducting until switch 96 is opened. As soon as SCR 97 conducts, a circuit is complete from hot line 91 to low-intensity alarm 106, and the alarm is energized and emits a low-level warning signal. Lowlevel alarm 106 may, for example, be a doorbell-type buzzer which emits and audible sound sufficiently loud to be heard by members of the household where the alarm is installed, if the members of the household are awake. Should a member of the household be awake and hear the low-level alarm, he need only go to the alarm unit and open switch 96. Then, he can warn the other members of the household that the alarm sounded, and unless the dangerous condition which caused the alarm to sound is readily apparent, a search can be instituted to determine why the alarm sounded.

In the event that the sensing alarm unit is not reset within seconds after the low-intensity alarm sounds, time delay relay 107 operates to close contacts 109 to actuate high-intensity alarm 111. The high-intensity alarm is of the type which emits a very loud piercing warning signal of a sufficiently high audible level to awaken any members of the household, if asleep, and for neighbors or others in the vicinity of the house to hear. Because of the high intensity of this alarm, which assures that a household will be awakened if the alarm sounds, the initial warning produced by the low-intensity alarm is highly desirable because of the effect of the high-intensity alarm on, for example, small children and sensitive people who may occupy the household. To turn off either or both alarms, it is merely necessary to open switch 96.

If there is a power failure, the alarm is actuated. This occurs because of the action of relay 114 which normally maintains contacts open so long as current to energize winding 113 is available between the chassis ground andjunction 112. This alternating current really keeps the contacts 115 open so long as this current exists, but allows the contacts to close when the winding 113 of the relay is deenergized. As soon as contacts 115 close, the sequence of operation is the same as previously described when SCR 97 is gated on, namely, low-intensity alarm 106 first sounds for a predetermined warning period, and if the unit is not turned off or reset then high-intensity alarm 111 sounds. Diode 86 prevents energizing relay 114 from battery 93.

The power for sounding low-intensity alarm 106 and highintensity alarm 111, when no power is available at junction 110, is provided by battery 93 because of the action of diode 90. As soon as the voltage at line 91 decreases somewhat below the terminal voltage of battery 93, diode 90 conducts, thereby providing positive voltage from terminal 92 of the battery at line 91. This voltage from the battery is then available to sound the several alarms, in the manner previously described. It will be noted that battery 93 will not discharge through the secondary winding of the transformer, when the alarm-sensing unit is deenergized because diode 86 blocks current flow from the battery through this secondary winding.

FIG. 5 shows schematically a second embodiment of a dangerous condition detecting unit according to this invention, and which can be used with the sensing and alarm units of FIGS. 1 and 4. In this embodiment, the detecting unit is provided with a two-prong plug 121 having plug prongs 122 and 123. It is supposed that the household receptacle is of the older type without a ground socket, and hence a separate wire 124 (similar to the wire 71 of the detecting device of FIG. 2) must be extended from the detector to a terminal 125 for connection to the metallic outlet box 126 to which the grounding conductor of the building is connected. However, a ground prong, like prong 53 of FIG. 2, can be provided if detecting unit 120 is used with a receptacle having a grounding socket. The circuitry of detector 120 includes wire 127 which extends from hot prong 122 to one side of a load in the form ofa lamp 128. Connected in series betiveen lamp 128 and ground wire 124 is a neon bulb 129 mounted on the detector housing 129' in such a manner that it is visible externally of the housing. In addition, there is a thermal switch 130 (like switch 78 previously described) in parallel with neon bulb 129.

To install detector 120, the terminal 125 ofground wire 124 is first grounded to outlet box 126. Next, the plug portion 121 of the detector is inserted in the household receptacle. If neon bulb 129 does not light, it is an indication that the plug portion ofthe detector is probably reversed in the receptacle and, correspondingly, this plug portion must be unplugged, reversed, and reconnected to the receptacle Then, neon bulb 129 will light, assuming of course, that the components of the detecting unit are sound, and it is assured that the detector is properly connected. If neon bulb 129 does not light, the house wiring is defective and the user apprised of this dangerous condition. Prong 123 merely provides a mechanical connection between the housing 129' of the detector 120 and the receptacle to support the detector at the receptacle.

In operation of the detector 120, neon bulb 129 is normally illuminated, receiving power for its energization through the high-resistance load provided by the lamp bulb 128. Hence, the homeowner can tell at a glance that the detector is operating properly, is properly connected, and that the wiring at the receptacle is proper. If thermal switch 130 closes, a connection is completed from prong 122 through lamp 128, and through ground wire 124 so that a small current passes through the grounding conductor at the fuse box or service entrance of the household in which the unit is installed. This small current is sufficient to activate the alarm system, in the manner previously described, to warn the household of the dangerous condition. The lamp bulb 128 is advantageously selected to have a rated voltage substantially less than the norma] 1 lO-volt household voltage, and is, for example, a 30-50 volt bulb. The advantage of using a low-voltage lamp bulb is that the lamp bulb will quickly burn out and hence, current through the ground wire will flow for only several seconds, a period of time insufficient to create a shock hazard, yet sufficient to activate the alarm system. With switch 120 closed, the homeowner can readily locate and replace that detecting unit which was actuated. Neon bulb 129 draws such a small current that the use of even or 12 of the detecting units 120 in a building will not conduct sufficient current through the ground wire to activate the alarm system because the sensing units 2 and 82' are so selected that a current flow in grounding bar 35 of about 100 milliamps is required to activate the alarms.

With reference to FIG. 6, there is shown an alternative arrangement to connect the various grounding conductors of the house wiring system to the fuse box, and which can be used in lieu of grounding bar 35. This alternative arrangement includes a transformer 135 having a primary winding 136 consisting of only several turns of very heavy wire (of sufficient gauge to safely carry the rated current of the building service entrance) wound around the core 137. The several grounding wires 138 of the household wiring system are connected to end 139 ofthe winding 136 and the other end of the winding is grounded to the service entrance housing 5 in the manner previously described for grounding bar 35, FIG. 1. Primary winding 136 serves the same purpose as the grounding bar 35.

In addition, there is a secondary winding 138 which is also wound on core 137. This secondary winding preferably includes a plurality of turns of small-gauge wire, and when current flows through the primary winding, a current is induced in the secondary winding to operate a latching switch may take the form of a latching relay 139 or a solid-state circuit including an SCR wired to latch into conduction after it is initially energized, such as the circuit described for sensing unit 82, FIG. 4.

While several preferred embodiments of the invention have been shown and described, it is to appreciated that detector 120 can be used with any of the systems described herein, and that, likewise, detector 50 can be used with any of these systems without departing from the scope of this invention. In households where the receptacles include the usual ground socket, a grounding arrangement including the grounding prong 53, as shown at FIG. 2, will be obtained. On the other hand, where the household receptacles do not have such a grounding plug, the ground wire 71 of FIG. 2 can be used, whereupon ground prong 53 is removed, or alternatively, detector with its ground wire 124 and terminal 125 will be provided. In addition, transformer can be used instead of grounding bar 35 in all the disclosed systems.

While several preferred embodiments ofa detecting unit including a thermal switch have been described in detail, it is to be understood that different detectors including smoke detectors and burglar alarms of all known types can, of course, be substituted in the system of this invention without departing from the basic scope of the invention.

What is claimed is: 1. For a building having a wiring system including hot wire, a neutral wire and a grounding conductor electrically connected at the service entrance, and grounding conductor traveling with the wiring, an alarm system comprising switch means adapted to be connected between the hot" wire of the wiring system and the grounding conductor at a location remote from the service entrance;

current-limiting means to limit current flow through said switch and grounding conductor;

means responsive to a predetermined condition to operate said switch means;

alarm means actuable to produce a warning signal; ad

alarm-actuating means associated with said grounding conductor at a location remote from said means responsive to a predetermined condition and operable in response to current in said grounding conductor to actuate said alarm means;

whereby, said alarm means is actuable independently of any other electrical loads carried by said wiring system without interfering with the operation of said electrical loads.

2. An alarm system according to claim 1 wherein said switch means comprises a switch having a movable contact and connected in series between the hot wire of the wiring system and said grounding conductor; and

said current-limiting means includes an impedance in series with said switch and grounding conductor.

3. An alarm system according to claim 1 wherein said means responsive to a predetermined condition is thermally responsive.

4; An alarm system according to claim 1 wherein said alarm-actuating means is associated with said grounding conductor at a location adjacent the service entrance ofthe building.

5. An alarm system according to claim 1 wherein said grounding conductor is a grounding wire grounded to the fuse box at the service entrance of the building;

said grounding wire travels with the wiring of the building and is connected to the electrical outlets of the building; and

said switch means is connected to one of the electrical outlets.

6. An alarm system according to claim 1 wherein said current-limiting means includes means to disconnect said hot" wire from said grounding conductor a predetermined period of time after said switch is operated to discontinue current flow through said grounding conductor.

7. An alarm system according to claim 6 wherein said current-limiting means includes a load in series with said switch means and grounding conductor, said load being selected to burn out several seconds after closing of said switch means;

whereby, current flow in said grounding conductor occurs for only several seconds.

8. An alarm system according to claim 6 wherein said load is a lamp bulb having a rated voltage substantially less than the voltage between the hot wire and grounding conductor. 9. An alarm system according to claim 1 wherein said location remote from the service entrance is a receptacle of the building wiring system; and said switch means is connected to the hot wire and the grounding conductor at the receptacle. 10. An alarm system according to claim 9 wherein said system further includes indicator means adjacent said receptacle to indicate proper connection of said switch means to said hot wire and grounding conductor, 11. An alarm system according to claim 10 wherein said indicator means is a neon bulb connected between said hot wire and said grounding wire. 12. An alarm system according to claim 10 wherein said neon bulb is illuminated whenever the switch means is properly connected and the building wiring system is sound. 13. An alarm system for a building comprising a service entrance having a grounded casing a wiring system including a plurality of circuits connected respectively to a plurality of electrical receptacles at different locations in said building, each of said circuits including a grounding conductor grounded at the receptacle, a neutral wire connected to one side of the receptacle, and a hot wire connected to the other side of the receptacle; a common conductor connected to said service entrance casing; means connecting said grounding conductors to said common conductor; current-sensing means associated with said common conductor between said grounding conductors and said service entrance for detecting a change of current in said common conductor, a plurality of detecting devices connected to the respective receptacles and each comprising a housing first conductor means extending between the housing and a receptacle and connected to the hot wire of the receptacle; second conductor means extending from the housing and connected to the grounding conductor at the receptacle, switch means connected between said first and second conductor means, current-limiting means in said housing and in series with said switch means, means responsive to a predetermined condition to operate said switch means; and alarm responsive to a change of current sensed by said current-sensing means to emit a warning signal. 14. An alarm system according to claim 13 wherein said switch means is carried by said housing ofa detecting device.

15. An alarm system according to claim 14 wherein said means responsive to a predetermined condition is carried by said housing ofa detecting device.

16. An alarm system according to claim 15 wherein said means responsive to a predetermined condition is thermally responsive.

17. An alarm system according to claim 13 wherein said first conductor means includes a plug prong insertable in a socket of the receptacle to connect the first conductor means to the hot wire and to support the housing at the receptacle.

8. An alarm system according to claim 17 wherein said second conductor means includes a ground plug prong insertable in a ground socket of the receptacle to connect the second conductor means to the grounding conductor and to further support the housing at the receptacle.

19. An alarm system according to claim 17 wherein said second conductor means includes a wire adapted to be connected to said grounding conductor at the receptacle. 20. An alarm system according to claim 13 wherein said detecting devices each include indicator means carried by said housing and connected between said first and second conductor means to indicate proper connection of said conductor means to said grounding conductor and said hot wire. 21. An alarm system according to claim 13 wherein said common conductor is the primary winding of a transformer, said primary winding having a high current-carrying capacity; and said sensing means includes the secondary winding of said transformer. 22. In an alarm system to apprise occupants of a dwelling about a dangerous condition, the combination comprising a dangerous condition detecting unit; a signal-sensing unit; means to transmit a signal from the detecting unit to the sensing unit in response to a sensed dangerous condition; a first low-intensity audible alarm connected to said sensing unit; a second high-intensity audible alarm connected to said sensing unit; means to actuate said low-intensity alarm in response to a signal from said sensing unit; and means to actuate said high-intensity alarm a predetermined period of time after the low-intensity alarm is actuated. 23. An alarm system according to claim 22 wherein said system further includes means to normally energize said sensing unit from the power supply ofthe dwelling; auxiliary power supply means, and means to actuate at least one of said alarms from said auxiliary power supply means in the event of failure of the dwelling power supply. 24. An alarm system according to claim 22 wherein said means to transmit a signal from the detecting unit to the sensing unit includes a grounding conductor of the housing wiring system. 25. An alarm system according to claim 24 wherein said dangerous condition sensing unit includes a switch operable to connect a hot" wire of the housing wiring system to said grounding conductor; and current-limiting means to limit the current in said grounding conductor when said switch is operated. 26. An alarm system according to claim 25 wherein said current-limiting means limits the amount of and duration of current flow in said grounding conductor. 

1. For a building having a wiring system including hot wire, a neutral wire and a grounding conductor electrically connected at the service entrance, and grounding conductor traveling with the wiring, an alarm system comprising switch means adapted to be connected between the ''''hot'''' wire of the wiring system and the grounding conductor at a location remote from the service entrance; current-limiting means to limit current flow through said switch and grounding conductor; means responsive to a predetermined condition to operate said switch means; alarm means actuable to produce a warning signal; ad alarm-actuating means associated with said grounding conductor at a location remote from said means responsive to a predetermined condition and operable in response to current in said grounding conductor to actuate said alarm means; whereby, said alarm means is actuable independently of any other electrical loads carried by said wiring system without interfering with the operation of said electrical loads.
 2. An alarm system according to claim 1 wherein said switch means comprises a switch having a movable contact and connected in series between the ''''hot'''' wire of the wiring system and said grounding conductor; and said current-limiting means includes an impedance in series with said switch and grounding conductor.
 3. An alarm system according to claim 1 wherein said means responsive to a predetermined condition is thermally responsive.
 4. An alarm system according to claim 1 wherein said alarm-actuating means is associated with said grounding conductor at a location adjacent the service entrance of the building.
 5. An alarm system according to claim 1 wherein said grounding conductor is a grounding wire grounded to the fuse box at the service entrance of the building; said grounding wire travels with the wiring of the building and is connected to the electrical outlets of the building; and said switch means is connected to one of the electrical outlets.
 6. An alarm system according to claim 1 wherein said current-limiting means includes means to disconnect said ''''hot'''' wire from said grounding conductor a predetermined period of time after said switch is operated to discontinue current flow through said grounding conductor.
 7. An alarm system according to claim 6 wherein said current-limiting means includes a load in series with said switch means and grounding conductor, said load being selected to burn out several seconds after closing of said switch means; whereby, current flow in said grounding conductor occurs for only several seconds.
 8. An alarm system according to claim 6 wherein said load is a lamp bulb having a rated voltage substantially less than the voltage between the hot wire and grounding conductor.
 8. An alarm system according to claim 17 wherein said second conductor means includes a ground plug prong insertable in a ground socket of the receptacle to connect the second conductor means to the grounding conductor and to further support the housing at the receptacle.
 9. An alarm system according to claim 1 wherein said location remote from the service entrance is a Receptacle of the building wiring system; and said switch means is connected to the hot wire and the grounding conductor at the receptacle.
 10. An alarm system according to claim 9 wherein said system further includes indicator means adjacent said receptacle to indicate proper connection of said switch means to said hot wire and grounding conductor,
 11. An alarm system according to claim 10 wherein said indicator means is a neon bulb connected between said hot wire and said grounding wire.
 12. An alarm system according to claim 10 wherein said neon bulb is illuminated whenever the switch means is properly connected and the building wiring system is sound.
 13. An alarm system for a building comprising a service entrance having a grounded casing a wiring system including a plurality of circuits connected respectively to a plurality of electrical receptacles at different locations in said building, each of said circuits including a grounding conductor grounded at the receptacle, a neutral wire connected to one side of the receptacle, and a hot wire connected to the other side of the receptacle; a common conductor connected to said service entrance casing; means connecting said grounding conductors to said common conductor; current-sensing means associated with said common conductor between said grounding conductors and said service entrance for detecting a change of current in said common conductor, a plurality of detecting devices connected to the respective receptacles and each comprising a housing first conductor means extending between the housing and a receptacle and connected to the hot wire of the receptacle; second conductor means extending from the housing and connected to the grounding conductor at the receptacle, switch means connected between said first and second conductor means, current-limiting means in said housing and in series with said switch means, means responsive to a predetermined condition to operate said switch means; and alarm responsive to a change of current sensed by said current-sensing means to emit a warning signal.
 14. An alarm system according to claim 13 wherein said switch means is carried by said housing of a detecting device.
 15. An alarm system according to claim 14 wherein said means responsive to a predetermined condition is carried by said housing of a detecting device.
 16. An alarm system according to claim 15 wherein said means responsive to a predetermined condition is thermally responsive.
 17. An alarm system according to claim 13 wherein said first conductor means includes a plug prong insertable in a socket of the receptacle to connect the first conductor means to the hot wire and to support the housing at the receptacle.
 19. An alarm system according to claim 17 wherein said second conductor means includes a wire adapted to be connected to said grounding conductor at the receptacle.
 20. An alarm system according to claim 13 wherein said detecting devices each include indicator means carried by said housing and connected between said first and second conductor means to indicate proper connection of said conductor means to said grounding conductor and said hot wire.
 21. An alarm system according to claim 13 wherein said common conductor is the primary winding of a transformer, said primary winding having a high current-carrying capacity; and said sensing means includes the secondary winding of said transformer.
 22. In an alarm system to apprise occupants of a dwelling about a dangerous condition, the combination comprising a dangerous condition detecting unit; a signal-sensing unit; means to transmit a signal from the detecting unit to the sensing unit in response to a sensed dangerous condition; a first low-intensity audible alarm connected to said sensing unit; a second high-intensity audible alarm connected to said sensing unit; means to actuate said low-intensity alarm in response to a signal from said sensing unit; and means to actuate said high-intensity alarm a predetermined period of time after the low-intensity alarm is actuated.
 23. An alarm system according to claim 22 wherein said system further includes means to normally energize said sensing unit from the power supply of the dwelling; auxiliary power supply means, and means to actuate at least one of said alarms from said auxiliary power supply means in the event of failure of the dwelling power supply.
 24. An alarm system according to claim 22 wherein said means to transmit a signal from the detecting unit to the sensing unit includes a grounding conductor of the housing wiring system.
 25. An alarm system according to claim 24 wherein said dangerous condition sensing unit includes a switch operable to connect a ''''hot'''' wire of the housing wiring system to said grounding conductor; and current-limiting means to limit the current in said grounding conductor when said switch is operated.
 26. An alarm system according to claim 25 wherein said current-limiting means limits the amount of and duration of current flow in said grounding conductor. 